This invention relates to the measurement of geophysical parameters of earth formations penetrated by a borehole and more particularly to propagation resistivity measurements at multiple transmitter frequencies and multiple transmitter receiver spacing. In addition the invention is directed toward the measurement of physical parameters of the borehole in addition to resistivity properties of the formation penetrated by the borehole. Measurements are made using an array of four transmitters and two receivers. Elements of the transmitter receiver array are longitudinally and symmetrically spaced along an elongated borehole instrument. Each transmitter induces an alternating voltage into the borehole and the earth formation in the vicinity of the borehole. The amplitudes and phase shifts of the signals produced by these induced alternating electromagnetic fields are measured by the receivers. These signals are effected by numerous formation, near borehole and borehole parameters. The measurements are combined to yield resistivity of the formation, parameters relating to the invasion of drilling fluids into the formation in the near borehole region, and physical characteristics of the borehole itself. The invention is directed toward, but not limited to, measurement while drilling (MWD) applications.
Multiple transmitter and receiver arrays have been employed in prior borehole induction measurements. The primary parameters of interest is the resistivity of the virgin or uninvaded formation from which the hydrocarbon saturation of the formation is determined. Symmetric pairs of transmitters and receivers have been employed to minimize or "cancel" the effects on the resistivity measurements of rapidly changing borehole parameters such as borehole diameter, eccentricity and rugosity. U.S. Pat. No. 4,899,112 to Brian Clark et al teaches a well logging technique in which electromagnetic propagation waves are used to measure formation resistivity at different radial depths of investigation. In addition, the cited patent teaches methods for determining the existence, location and properties of beds and caves, and also teaches a method for determining changes in the size of the borehole. The measurements are based upon the observation that phase and amplitude apparent resistivity measurements, made at a given transmitter frequency and a given transmitter-receiver spacing, exhibit different depths of investigation. The teachings of Clark do not provide a method for the user to select from a "menu" the parameters of interest as does the present invention. There are other advantages of the present invention that will be emphasized in the following discussions. Multiple transmitter-receiver spacing have also been employed in the prior art to obtain measurements into the formation of varying radial depths of investigation. Combining such measurements tends to minimize borehole effects as well as yield information concerning the radial extent of the invasion of drilling fluid into the virgin formation. Invasion measurements can be related to the permeability of the formation which, in turn, is related to the producibility of fluids contained within the formation rock matrix. Again, no attempts have been made in the prior art to obtain quantitative measures of physical characteristics of the borehole in conjunction with measures of electromagnetic properties of the formation. Multiple transmitter frequencies have also been applied in the prior art to enhance and separate electromagnetic properties of the formation such as resitivity and dielectric constant, obtaining varying effecting radial depths of investigation and to a lesser extent to minimize borehole effects. Once again, contributions from the borehole effects have not been quantified and related to the physical condition of the borehole.